The tissue is defined as group of cells which has common origin and development to perform a particular function. Epithelial tissue- Epithelial tissue is a group of cells which are arranged in layer(s) to form external or internal covering of the body or organs.
(1) Epithelial tissue covers the free surface of the organs.
(2) It is derived from ectoderm, mesoderm and endoderm.
(3) It consists of compactly arranged cells which lie in one or more layers.
(4) There is little amount of intercellular ground substances between the cells. This ground substances contains hyaluronic acid and Ca++ salts which is secreted by the cells.
(5) The Epithelia set on a thin, non-cellular, gelatinous basement membrane. The basal region of epithelial tissue consists of 3 components. They are basal lamina, reticular fibres, proteins and polysaccharides. The basal lamina is secreted by overlying epithelial cells where as reticular fibres proteins and polysaccharides are secreted by underlying connective tissue or lamina propria.
(6) The Epithelial cells lack blood vessels but they get food and oxygen through basement membrane.
(7) The plasm membranes of adjacent epithelial cells are held together by interdigitations, tight junction, desmosomes and intercellular bridges.
Function- Its main function is to protect the underlying tissue and it also helps in absorption, secretion, excretion, filtration in different organs.
On the basis of structure and function, Epithelial tissue is of 2 types :(A) Covering epithelium,(B) Glandular epithelium.
(A) Covering Epithelium:
It covers both external and internal free surfaces. It is of 3 types. They are simple epithelium, compound epithelium, and transitional epithelium.
(1) Simple Epithelium: - It is made up of a single layer of cells. It is of following types-
(i) Squamous Epithelium:
It is composed of a single layer of scale-like flat cells. Hence the name is squamous. It is also called pavement epithelium. The squamous epithelium with wavy cell outlines is called tesseslated epithelium.
It forms endothelium in blood vessels, peritoneum in coelom, lung alveoili. It is also found in Bowman’s capsule and Henle’s loop of nephron.
It protects the underlying tissue. It has got filtering function. It allows easy passage of liquid or gases through it.
(ii) Cubical Epithelium:
It is made up of a single layer of cubical cells having same dimension on each side and placed on basement membrane.
It forms germinal epithelium of ovary and inner part of digestive and salivary glands.
Protection, secretion, storage of glucogen and metal ions are the functions of this tissue.
(iii) Columnar Epithelium:
It is made up of elongated tall pillar-like cells.t nuclei are placed at the basal part.
In alimentary canal and proximal convoluted tubules of nephron, their free boarders are longitudinally striated. Hence called brush border epithelium. Under electron microscope the brush borders appears to be finger like protections called microvilli. Microvilli increase surface area for absorption of food materials.
It is found in lining stomach, intestine, gall bladder, proximal convoluted tubules of nephron.
Its chief function is absorption of digested food and secretion.
(iv) Ciliated Epithelium:
Here each cell bears 20-30 cilia on its free surface. Each cilium arises from a basal granule lying in the cytoplasm. The membrane of a typical cilium is continuous with cell membrane.
It is found in lining of nose, larynx, trachea, small bronchi, bronchioles, fallopian tubes.
Function – Beating of cilia propels mucous, liquid, suspended particles, ova etc.
(v) Pseudostratified Epithelium:
It is made up of single-layered cells. It looks many layered due to unequal height of the cells. They are usually ciliated.
It is found in urethra and trachea.
It conducts different substances like ciliated epithelium.
(vi) Modified epithelium:
(a) Germinal epithelium- It produces gametes in ovary and testes.
(b) Sensory epithelium- In retina and taste buds it receives and conveys stimuli.
(c) Myoepithelium-In salivary glands and their ducts, the cells contain actin and myosin filaments and contract to release secretory products from the glands.
(d) Pigmented epithelium- In retina the cells contained coloured melanin pigment granules.
(e) Ependymal epithelium- It forms lining in the cavities of brain and spinal chord. They are ciliated.
(f) Cuticular epithelium- It secretes cuticle, the tough resistant non-cellular protective layer of the body in invertebrates.
1. Compound Epithelium:
It is made up of many layers of cells. The cells and innermost layers are living and divide mitotically to replace the outermost dead cells.
It protects the underlying tissue from mechanical pressure, friction and injury.
It is of following types:
(i) Stratified squamous cornified – The outermost cells become dead and hard due to deposition of scleroprotein keratin. The innermost dividing layer, stratum germinativum replaces the outermost lost cells, e.g. skin, hair, horn, hoof and nail.
(ii) Stratified squamous non- keratinized- Here the superficial cells are not keratinized, e.g. buccal cavity, pharynx, vagina, cervix, anal canal etc.
(iii) Stratified cuboidal- The cells of outermost layer are cubical in shape, e.g. Graafian follicle.
(iv) Stratified columnar- The outermost cells are columnar cells are columnar in shape, e.g. part of urethra, epiglottis, etc.
(v) Stratified ciliated- The outermost cells bears cilia, e.g. larynx.
2. Transitional Epithelium:
It is found between the single layered simple epithelium and many layered Stratified epithelium. Hence, the name is transitional. It consists of 3 or 4 layers of cells. The cells of superficial layer are irregularly quadrihedral. The next layer consists of pyriform cells which lie on one or two layers of polyhedral cells.
It is found in the pelvis of kidney, ureter, urinary bladder and upper part of urethra.
It prevents reabsorption of excreted materials back to the system. It prevents in drawing of water from blood and tissues by the higher osmotic pressure of urine.
(B) Glandualr Epithelium:
A gland is an organ of secretion. It is classified into two unicellular gland and multicellular gland. Goblet cells are unicellular gland and they secret mucous. The multicellular glands are classified as shown in table 1.
Types of glands on the basis of mode of secretion:
1. Apocrine Gland - Secretions are released from the apical part of the cell, e.g. mammary glands.
2. Holocrine Gland - Entire cell is discharged during secretion, e.g. sweat gland, salivary gland.
3. Merocrine Gland - Secretions are released through the plasms membrane by exocytosis e.g. sweat gland, salivary gland.
On the Basis of nature of secretion:
(a) Serous Gland- It secretes watery substances, e.g. sweat gland, intestine gland, parotid gland.
(b) Mucous Gland- It secretes a viseous and slimy secretions, e.g. goblet cell, some gastric gland and colon glands.
(c) Mixed Gland- It secretes both serous and mucous secretions, e.g. pancreas and most gastric glands.